Cancer remains one of the most prolific killers in industrialized countries. While surgery, radiation and chemotherapy are effective in the treatment of some cancers, many others are resistant to such therapies. This is evidenced by the high mortality rate; approximately 1 in 4 deaths in the United States are cancer-related. Lung and stomach cancer are particularly deadly, with survival rates averaging 15% after five years. High mortality rates are due in part to their resistance to treatment methods; for example, both stomach and lung cancers are largely insusceptible to chemotherapy treatment. Thus, there is a constant need to develop improved cancer therapies.
In 2001, several groups used a cloning method to isolate and identify a large group of “microRNAs” (miRNAs) from C. elegans, Drosophila, and humans (Lagos-Quintana et al., 2001; Lau et al., 2001; Lee and Ambros, 2001). Several hundreds of miRNAs have been identified in plants and animals—including humans—which do not appear to have endogenous siRNAs. Thus, while similar to siRNAs, miRNAs are nonetheless distinct.
MiRNAs thus far observed have been approximately 21-23 nucleotides in length and they arise from longer precursors, which are transcribed from non-protein-encoding genes (Carrington et al., 2003). The precursors form structures that fold back on each other in self-complementary regions; they are then processed by the nuclease Dicer in animals or DCL1 in plants. MiRNA molecules interrupt translation through precise or imprecise base-pairing with their targets.
MiRNAs are involved in gene regulation. Some miRNAs, including lin-4 and let-7, inhibit protein synthesis by binding to partially complementary 3′ untranslated regions (UTRs) of target mRNAs. Others, including the Scarecrow miRNA found in plants, function like siRNA and bind to perfectly complementary mRNA sequences to destroy the target transcript (Grishok et al., 2001).
Research on miRNAs is increasing as scientists are beginning to appreciate the broad role that these molecules play in the regulation of eukaryotic gene expression. The two best understood miRNAs, lin-4 and let-7, regulate developmental timing in C. elegans by regulating the translation of a family of key mRNAs (Pasquinelli, 2002). Several hundred miRNAs have been identified in C. elegans, Drosophila and mouse. More than thousand miRNAs have been discovered in humans. As would be expected for molecules that regulate gene expression, miRNA levels have been shown to vary between tissues and developmental states.
MicroRNAs and Cancer
There is growing realization that miRNAs, in addition to functioning as regulators of development, can act as oncogenes and tumor suppressors (Akao et al., 2006; Esquela-Kerscher and Slack, 2006; He et al., 2005). Data suggests the dysregulation of miRNA expression in cancer cells (Cho, 2009; Galasso et al., 2010; Garzon et al, 2006; Leite et al., 2009; Li et al., 2010; Ohlsson Teague et al., 2009; Tie et al., 2010; Varnholt et al., 2008). Moreover, altered expression of specific miRNAs has been demonstrated to promote tumorigenesis. Thus, these miRNA expression changes are informative for cancer classification and prognosis.
Events leading to the development of cancer from normal tissue have been well characterized, and a necessary step in this process is the dysregulation of cell cycle progression that facilitates the propagation and accumulation of genetic mutations. Within each cell, elaborate machinery exists to halt cell cycle progression in response to various stimuli, including DNA damage. Such regulation provides time for DNA repair prior to its replication and cell division, hence preserving the integrity of the genome. Multiple pathways lead to cell cycle arrest; however, the p53 tumor suppressor pathway has been shown to lead to both G1 and G2M arrest (Vousden et al., 2007; Taylor et al., 2001; Brown et al., 2007). Although a number of players in this pathway have been identified and characterized, the precise mechanism by which DNA damage leads to cell cycle arrest remains only partially understood.
Cell cycle arrest in response to DNA damage is an important anti-tumorigenic mechanism. MiRNAs have been shown to play regulatory roles in cell cycle progression. In doing so, miRNAs regulate biological processes including cell growth, differentiation and death (Bartel et al., 2004). Insight has been gained into the miRNA-mediated cell cycle regulation by identifying target transcripts of respective miRNAs (Carleton, 2007; Johnson et al., 2007; Ivanovsaka, I., et al., 2008). For example, miR-34a is induced in response to p53 activation and mediates G1 arrest by down-regulating multiple cell cycle-related transcripts.
While certain miRNAs exert their cell cycle effect through targeting transcripts, other miRNAs do so through cooperatively down-regulating the expression of multiple cell cycle-related transcripts (He et al., 2007; Linsley et al., 2007). In addition to their effects on the cell cycle, these miRNAs and their family members are aberrantly expressed in human cancers (Linsley et al., 2007; Calin et al., 2006; Takamizawa et al., 2004; Inamura et al., 2007; Cimminio et al., 2005; Ota et al., 2004; He et al., 2005).
Cancer causes one in every four US deaths and is the first leading cause of death among Americans. Despite extensive research into the development of therapies, current neoplasia treatments are woefully ineffective. Many mechanisms of miRNA regulation, including their targets and roles in neoplastic transformations, have not yet been investigated. Thus, there is an unfulfilled need for improved compositions and methods for the treatment or prevention of neoplasia.